This invention relates to an apparatus for forming a thin film and more particularly to such an apparatus by plasma chemical vapor deposition (CVD) suitable for use in the production process for a magnetic disc serving as a memory medium for a hard disc memory device.
The magnetic disc for a hard disc memory device is usually produced by forming a magnetic layer on the surface of a disc such as a glass disc and further providing a protective layer thereon. An apparatus for forming a thin film by plasma CVD may be used to form a carbon film as a protective layer on both surfaces of a disc at the same time. The carbon film comprises amorphous carbon, and such a film formed by plasma CVD is very hard and has superior characteristics such as resistance against abrasion, corrosion, peeling, voltages and insulation. Thus, carbon films are most suitable as a protective layer for a magnetic disc. The hardness of these films can be controlled by adjusting the bias voltage applied to the substrate at the time of the plasma CVD.
With a prior art apparatus for forming a thin film by plasma CVD, however, a negative charge-up may take place when a bias voltage is applied to the substrate, resulting in an abnormal discharge and causing particles to be emitted from the chamber walls and to become attached to the substrate. There may also occur peeling on the substrate surfaces.